Conventionally, for example, in a vehicle equipped with a gasoline engine, a heat pump is used for cooling, while the waste heat of the engine is used for heating. Typically, cooling is performed while circulating air inside the cabin, and heating is performed while introducing less humid outside air thereinto.
In recent years, hybrid vehicles using less waste heat of engines and electric vehicles unable to use waste heat of engines have become increasingly popular. To keep pace with this trend, vehicle air conditioners using heat pumps not only for cooling but also for heating have been developed.
When a heat pump is used for heating, it is preferable, from the viewpoint of reducing power consumption, to perform heating while circulating the air inside the cabin rather than introducing outside air. However, since the air inside the cabin contains moisture derived from the occupants, if the air inside the cabin is circulated during heating, the moisture may cause fogging of the windows. In view of this, a vehicle air conditioner is required to have a function of dehumidifying the air inside the cabin also during heating.
For example, Patent Literature 1 discloses an air conditioner for an electric vehicle as shown in FIG. 13. This air conditioner includes a compressor 101 driven by an electric motor, an outside heat exchanger 102, an expansion valve 103, a heat absorbing indoor heat exchanger 104, and a four-way valve 105 for forming a loop connecting these components 101 to 104 in this order during a cooling operation. A heat releasing indoor heat exchanger 106 and an expansion valve 107 are connected to the other two ports of the four-way valve 105 so as to form another loop. Air inside the cabin is supplied to the heat absorbing indoor heat exchanger 104 by a fan 110. A high temperature and high pressure gas refrigerant compressed in the compressor 101 is introduced to the heat releasing indoor heat exchanger 106 during a heating operation, and this heat releasing indoor heat exchanger 106 is placed downstream of the heat absorbing indoor heat exchanger 104 in the direction of the air flow formed by the fan 110.
In the above-described air conditioner, during the cooling operation, the gas refrigerant compressed in the compressor 101 flows through the four-way valve 105 in a direction of a solid line arrow, changes to a liquid refrigerant through heat exchange with outside air in the outdoor heat exchanger 102, and then is throttled through the expansion valve 103 and adiabatically expanded. The adiabatically expanded liquid refrigerant changes to a gas refrigerant through heat exchange with cabin circulating air supplied by the fan 110 in the heat absorbing indoor heat exchanger 104, and returns to the compressor 101.
On the other hand, during the heating operation, the gas refrigerant compressed in the compressor 101 flows through the four-way valve 105 in a direction of a dotted line arrow, changes to a liquid refrigerant through heat exchange with cabin circulating air supplied by the fan 110 in the heat releasing indoor heat exchanger 106, while heating the circulating air. The liquid refrigerant leaving the heat releasing indoor heat exchanger 106 passes through the expansion valve 107, the four-way valve 105, the outdoor heat exchanger 102, the expansion valve 103, and the heat absorbing indoor heat exchanger 104 in this order and returns to the compressor 101. The liquid refrigerant is throttled through the expansion valve 107 and adiabatically expanded, and then changes to a gas refrigerant through heat absorption in the outdoor heat exchanger 102 and in the heat absorbing indoor heat exchanger 104, while cooling the circulating air in the outdoor heat exchanger 102. As a result, condensation water is generated on the surface of the heat absorbing indoor heat exchanger 104. The condensation water drops into a drain pan 108 and is exhausted through a drain pipe 109 to the outside of the cabin.
In this way, the air inside the cabin circulated by the fan 110 is cooled and dehumidified in the heat absorbing indoor heat exchanger 104, but it is heated in the heat releasing indoor heat exchanger 106. This makes it possible to perform heating while dehumidifying the air inside the cabin.